The range of applications of liquid crystal displays (hereinafter, referred to as “LCD”) is gradually broadening due to characteristics such as light weight, thinness and low power consumption. The LCD is used in office automation equipment, audio/video devices and similar applications. The LCD displays a desired picture on a screen by controlling the amount of transmitted light in accordance with a video signal applied to a plurality of control switches which are arranged in a matrix configuration.
The LCD needs a light source like a backlight because it is not a self-luminous display device. A cold cathode fluorescent lamp (hereinafter, referred to as “CCFL”) may be used as the light source in the backlight.
A CCFL is a light source tube using cold emission phenomenon; electrons are emitted because a strong electric field is applied to the surface of a cathode, so that low heat generation, high brightness, long life span and full colorization are obtained. The CCFL can be of light guide system, direct illumination system or reflection plate system, and a light source tube is adopted in accordance with the design requirements of the LCD.
The CCFL uses an inverter circuit to produce a high-voltage power from a low voltage DC power source.
Referring to FIGS. 1 and 2, the lamp driving apparatus of an LCD includes a lamp housing 10 into which a plurality of lamps 12 are put; an inverter part 22 with a plurality of inverters for supplying an output voltage to each of the lamps 12; a first printed circuit board 20 on which the inverter part 22 is mounted; a lamp protector 32 for protecting each of the lamps 12; and a second printed circuit board 30 on which the lamp protector 32 is mounted.
The lamp housing 10 has a space provided for receiving the lamps and is disposed on a main support (not shown).
Each lamp receives the lamp output voltage from the inverter part 22 and illuminates a liquid crystal display panel (not shown) with visible light.
The first printed circuit board 20 is arranged at one side of the support main (not shown) and folded to the direction of the rear surface of the support main.
The second printed circuit board 30 is arranged at one side of the support main (not shown) and folded to the direction of the rear surface of the support main.
As shown in FIG. 2, each inverter 24 constituting the inverter part 22 includes a switching circuit 26 to switch a voltage from a voltage source Vin in response to a switching control signal, and a transformer 28 to convert the voltage supplied by switching of the switching circuit 26 into an output voltage.
The switching circuit 26 switches the voltage from the voltage source Vin to the transformer 28 in response to the switching control signal from a pulse width modulator PWM 34. For this purpose, the switching circuit 26 includes at least one switching device.
The transformer 28 includes a primary winding wire connected to the switching circuit 26 and a secondary winding wire connected to the lamp 12. Both ends of the primary winding wire are connected to the switching circuit 26 and one end of the secondary winding wire is connected to a first electrode terminal of the lamp 12, and the other end is connected to a ground (GND). The transformer 28 converts the voltage supplied to the primary winding wire by a winding ratio of primary and secondary winding wires and induces it in the secondary winding wire. The voltage induced on the secondary winding wire is supplied to the lamp 12 through a first electrode terminal and lights the lamp 12.
The lamp protector 32 includes an open lamp protector OLP 36 to detect the presence or absence of the lamp 12 by the output voltage of the lamp 12; an over voltage protector OVP 38 to detect the voltage supplied to the electrode part of the lamp from the transformer 28; and a pulse width modulator 34 for switching the switching circuit 26 in response to a feedback signal FB2 from the over voltage protector 38.
The open lamp protector 36 detects the presence or absence of the lamp 12 by the output voltage of the lamp 12 to control the pulse width modulator 34. That is, in the case that the lamp 12 is not present, the open lamp protector 36 generates a feedback signal FB1 corresponding to the detected detection signal. In this circumstance, the pulse width modulator 34 inhibits the switching circuit 26 such that the voltage from the voltage source Vin is not supplied to the transformer 28, in accordance with a feedback signal FB1 from the open lamp protector 36. Thus, in case that the lamp 12 is not present, the inverter part 22 does not supply the voltage to the lamp 12.
The over voltage protector 38 detects the voltage supplied to the electrode part of the lamp 12 from the transformer 28 to control the pulse width modulator 34. That is, as shown in FIG. 3, when an over voltage V2 of not less or more than voltage levels OVP1, OVP2, respectively, which would cause damage to the lamp 12 is supplied to the electrode part of the lamp 12 from the transformer 28, the over voltage protector 38 generates the feedback signal FB2 corresponding to the detected detection signal and supplies the generated signal to the pulse width modulator 34. In this circumstance, the pulse width modulator 34 controls the switching period of the switching circuit 26 by the feedback signal FB2 from the over voltage protector 38 to reduce the voltage supplied to the primary winding wire of the transformer 28 from the voltage source Vin. Thus, the voltage supplied to the lamp 12 from the secondary winding wire of the transformer 28 is reduced to V3 to prevent the lamp 12 from being damaged.
The pulse width modulator 34 controls the switching period of the switching circuit 26 in response to the feedback signal FB2 from the over voltage protector 38 and the feedback signal FB1 from the open lamp protector 36. That is, the pulse width modulator 34 controls the voltage supplied to the transformer 28 by controlling the switching period of the switching device, which constitutes the switching circuit 26 in response to the feedback signals FB1, FB2.
In the lamp driving device of the LCD, the lamp lighting voltage and the operating voltage required by the lamp 12 is directly proportional to the length of the glass tube of the lamp 12. As the voltage increased in this way, as shown in FIG. 4, it can generate an undesired mis-discharge between adjacent lamps 12 and render the output voltage of the inverter 24 unstable.
Hence, the lamp driving apparatus of the existing LCD designs can cause the lamps 12 to be damaged because no protective circuit is provided for responding to the mis-discharge that occurs between the adjacent lamps 12.